This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The pathogenesis of HIV/SIV encephalitis (HIVE/SIVE) remains incompletely understood, but is associated with alterations in the blood brain barrier (BBB). Heretofore, it has not been possible to easily determine if an individual has HIVE/SIVE before post mortem examination. We have examined serum levels of the astroglial protein S100b in SIV-infected macaques and show that it can be used to predict which animals will develop SIVE. We also found that increased S100b protein in serum correlated with decreased expression of the tight junction protein zonula occludens-1 on brain microvessels. Further, the decrease in zonula occldens-1 expression was spatially related to SIVE lesions and perivascular deposition of plasma fibrinogen. Together these data indicate that SIVE lesions are associated with vascular leakage that can be monitored by S100b protein in the periphery. The ability to simply and prospectively monitor the development of SIVE will greatly facilitate studies of the neuropathogenesis of AIDS.